Ultimate Tag Warrior (UTW) is a tagging plugin for wordpress. It allows storing a set of keywords (or as current slang dictates, tags) against posts that behave like categories – the tags can be displayed on a post, you can go to the archive page for a tag, and there are RSS/atom feeds available. Unlike categories, you can also view archive pages and feeds for sets of tags such as the posts which are tagged with both “photograph” and “food”, for example.